The distinction between terminal and penultimate copolymerization models

An examination of the usual terminal and penultimate mechanisms of copolymerization shows that, for a great many comonomer pairs, copolymer composition data are not sufficient to choose one model over the other. The reactivity ratios based on one copolymerization model can be derived from the reactivity ratios of the other model. Predicted copolymer composition, based on both models, will agree with the observed experimental data within the precision of the usual methods used for copolymer analysis. Hypothetical cases and examples from the literature all show this behavior. The literature comonomer systems were α‐methylstyrene–acrylonitrile, and styrene with maleic anhydride, citraconic anhydride, and acrylonitrile. Polymer structure, however, predicted by the two mechanisms do differ appreciably in detail. In particular, predictions of the sequential arrangements of the monomers in the copolymers produced are different for the terminal and penultimate mechanisms. It would seem that the development of experimental procedures for determining the sequential arrangement of the comonomers will be necessary before one can easily and definitely distinguish between terminal and penultimate copolymerization models for a particular system.